1. Field of the Invention
This invention relates generally to propulsion chambers, sometimes called blast nozzles used in cleaning metal surfaces with abrasive particles propelled by a combination of liquid and air, and particularly to a new and improved blast nozzle apparatus in which high pressure water is injected substantially coaxially with a flow of a mixture of air and abrasive entering the apparatus for increased performance and service life.
2. Description of the Prior Art
In order to completely clean a corroded metal surface down to a "white" metal condition so that such surface can be painted to preserve the metal against deterioration, it has become common practice to use various abrasive blasting techniques where abrasive particles are propelled against the metal surface in order to dislodge the oxides, previously applied coatings, scale and other contaminants. One cleaning technique has involved a two-step process consisting of dry blasting to apparent white metal, followed by high pressure water blasting to remove contaminants and oxides from microscopic pits in the surface. Another more efficient process has involved a high pressure water jet of the wet jet abrasive blaster type that accelerates abrasive particles against the surface, propelled by both a high pressure, high velocity water jet and air, so that cleaning can be accomplished in a single step. The single step process is preferred because iron oxide "caps" on surface pits which may contain water soluble iron salts do not have sufficient time to form, as in the case of a two-step process, so that the salts are flushed out of the pits to provide a truly clean surface.
In most any water-wetted abrasive blasting operation, the principle problems are slow cleaning rate, i.e., "performance" and early erosion of the nozzle or propulsion chamber body by abrasive flow, which prevents the maintenance of a stable flow pattern. Erosion and wear within the propulsion chamber or at a location within the outlet nozzle member results in a concentration of the blast of abrasive particles, which will reduce productivity and cause wear through a nozzle body in a relatively short period of time, thus rendering the nozzle inoperative. Although there appears to be no way to prevent erosion and wear altogether in this type device, the invention described below provides a propulsion chamber design with remarkably reduced wear characteristics and improved cleaning or production rates as compared to prior devices.